Electrostatic actuator for micromechanical systems

ABSTRACT

A method and apparatus are described that may be used to provide decoupled rotation of structures about different pivot points. The apparatus may include one or more fixed blades mounted to a frame or substrate, one or more movable blades mounted to each structure to be moved, and flexures on which the structures are suspended. Separate movable blades may be provided for each degree of freedom. When voltage is applied between the fixed and movable blades, the electrostatic attraction generates a force attracting movable blades toward blades that are fixed relative to the moveable blades, causing a structure to rotate about the flexures. The angle of rotation that results may be related to the size, number and spacing of the blades, the stiffness of the flexures and the magnitude of the voltage difference applied to the blades. The blades are fabricated using deep silicon etching.

REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. Section 119(e) ofU.S. Provisional Application No. 60/179,912, filed Feb. 3, 2000,entitled Electrostatic Actuator for Micro Electro-Mechanical SystemsWith Method of Manufacture, and Product Using Same.

FIELD OF THE INVENTION

This invention relates to the field of electrostatic actuators and, inparticular, to microelectromechanical (MEM) electrostatic actuators.

BACKGROUND

Prior parallel-plate actuators, such as the example illustrated in FIGS.1A (top view), 1B (side view), and 1C (side view), are typicallydesigned with gaps 13 that are significantly larger than the strokerange of the actuator. When a voltage is applied between two electrodeplates 15 and 10, an attractive force is produced between the electrodeplates that rotates plate 10. Because the maximum rotation is determinedby the separation, or gap 13, between the two electrode plates 15 and10, there must be a large separation in order to obtain a largedeflection. The gap 13 needs to be much larger than absolutely necessaryfor the physical movement of electrode plates 15 and 10, because if theelectrodes approach too closely to each other (e.g., less than about ⅓of gap 13), a point of instability is reached where the electrodes 15and 10 may snap together.

Because the force produced by a parallel-plate actuator is proportionalto (voltage/gap)², as gap 13 increases, the voltage must also go up withthe square of the distance in order to achieve the same force. With themovement of the structure, electrode plates 15 and 10 do not remainparallel to each other and gap 13 between them decreases. Hence, thevoltage required to move electrode plates 15 and 10 a given distance ishigh, nonlinear, and constantly changing. This may require more complexelectronics to control the actuator that may be difficult and costly tobuild. Also, the use of a large gap may result in cross-talk betweenadjacent actuators in an array.

Moreover, on the extremely small scale of these actuators, problems areintroduced by the need to run conductors for the voltages very closetogether. With higher voltages, interactions between conductors are hardto avoid and in extreme cases, arcing between conductors will occur,leading to damage to the device. Current parallel plate actuators havinga useful range of movement typically require voltages of 300 volts orhigher.

U.S. Pat. No. 5,536,988 entitled Compound Stage MEM Actuator SuspendedFor Multidimensional Motion discloses the use of interlocking combfingers as X-Y axis actuators for nested stages of MEMs devices. Thelevitation force produced by comb fingers can also be use to generatetorsional actuators. Nevertheless, the primary limitation of combfingers is on the stroke range. The levitation force produced by combdrives is limited to approximately the same distance that the combfingers are spaced. This typically makes deflections greater that 5 to10 microns (μm) very difficult. Deflections greater than 50 μm may beneeded, however, for mirror actuator applications, which may not bepossible to achieve with the comb finger actuators.

SUMMARY OF THE INVENTION

An apparatus and method of actuation are described. For one embodiment,the apparatus may include a stage having a surface and a first bladecoupled to the stage with the first blade extending perpendicular to thesurface of the stage. The apparatus may also include a frame having asurface and a second blade coupled to the frame. The stage is pivotallycoupled to the frame. The second blade extends perpendicular to thesurface of the frame and is parallel with the first blade.

For one embodiment the stage may be pivotally coupled to the frame by atorsional flexure. By applying a voltage difference between the firstand the second blades, an electrostatically generated torque will causethe stage to rotate to an angle related to the magnitude of the voltagedifference.

For another embodiment, the apparatus may include a central stage, amovable frame, and a fixed frame. The central stage may be coupled tothe movable frame by a first torsional flexure, and the movable framemay be coupled to the fixed frame by a second torsional flexure,perpendicular to the first. Blade actuators may be attached to thecentral stage and movable frame to tilt the central stage with respectto the movable stage. Blade actuators may be attached to the movableframe and the fixed frame to tilt the movable stage with respect to thefixed stage. A mirror may be attached to the central stage.

Methods for fabricating a microelectromechanical apparatus are alsodescribed. For one embodiment, first trenches are formed in a first sideof a substrate. A layer of dielectric material is formed on the firstside of the substrate. The first trenches are filled with the dielectricmaterial to provide electrical isolation. A masking layer is patternedon a second side of the substrate that is opposite to the first side ofthe substrate. Vias are formed on the first side of the substrate. Thefirst side of the substrate is metallized. Second trenches are formed onthe first side of the substrate to define structures. The second side ofthe substrate is deeply etched to form blades. Etching is performed torelease the structures.

Additional features and advantages of the present invention will beapparent from the accompanying drawings and from the detaileddescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements and in which:

FIG. 1A is a top view illustrating a prior art pivoting structure thatuses parallel plate actuation.

FIG. 1B is a side view illustrating the prior art parallel plateactuation structure of FIG. 1A.

FIG. 1C is a side view illustrating the parallel plate actuationstructure of FIG. 1B with an electrostatic plate activated.

FIG. 2A illustrates one embodiment of an actuator with a stage parallelto a frame.

FIG. 2B shows the actuator of FIG. 2A with the stage tilted with respectto the frame.

FIG. 3A is a perspective view illustrating one embodiment of a bladeactuator.

FIG. 3B is a perspective view illustrating actuation of the bladeactuator of FIG. 3A.

FIG. 3C is a side view illustrating the actuation of the blade actuatorof FIG. 3A.

FIG. 3D is a top view illustrating an alternative embodiment of a bladeactuator.

FIG. 3E is a top view illustrating another embodiment of a bladeactuator.

FIG. 4 illustrates an alternative embodiment of a blade for an actuator.

FIG. 5A is a top view illustrating one embodiment of a multiple stageactuator.

FIG. 5B illustrates one embodiment of torsional flexures.

FIG. 5C illustrates an alternative embodiment of torsional flexures.

FIG. 6 illustrates one embodiment of the underside of the multiple stageactuator 500 of FIG. 5A.

FIG. 7A illustrates one embodiment of an actuator array.

FIG. 7B illustrates one embodiment of interconnect metallizations.

FIG. 8 illustrates a mirror cell.

FIGS. 9A through 9K show cross sections associated with one method offabricating a mirror cell.

FIG. 9A is a cross section of a silicon wafer ready to be processed.

FIG. 9B shows a portion of the wafer with a masking layer, a photoresistlayer, and an opening to the silicon surface of the wafer.

FIG. 9C shows an isolation trench formed in the silicon wafer.

FIG. 9D shows a portion of the wafer with a dielectric layer on the topsurface of the silicon wafer and on the sidewalls and bottom of theisolation trench.

FIG. 9E shows the portion of the wafer after planarization of thedielectric layer.

FIG. 9F shows isolation trenches on the top of the wafer and a maskinglayer for blades on the bottom of the wafer.

FIG. 9G shows metallization on the top of the wafer.

FIG. 9H shows trenches on the top of the wafer.

FIG. 9I shows the blades that result from deep silicon etching.

FIG. 9J shows a base wafer bonded to the wafer containing the blades.

FIG. 9K shows the wafer after a release etch separates portions of thestructure and after the attachment of the lid wafer.

FIGS. 10A through 10E show cross sections associated with another methodof fabricating a mirror cell.

FIG. 10A shows a cross section of the wafer after the blades arefabricated using deep silicon etching and after a base wafer has beenfusion bonded to the wafer containing the blades.

FIG. 10B shows the wafer after a portion of the top of the wafer hasbeen removed using polishing and after isolation trenches, vias, metalinterconnects, and mirror metallization have been formed.

FIG. 10C shows trenches on the top of the wafer.

FIG. 10D shows a cross section of the wafer after a release etchseparates portions of the structure.

FIG. 10E shows a glass lid attached to the wafer.

FIG. 11 shows a cross section of a silicon-on-insulator (“SOI”) waferthat includes blades formed by deep silicon etching.

FIGS. 12A through 12E show cross sections associated with yet anothermethod for fabricating a mirror cell.

FIG. 12A shows a cross section of a silicon device wafer having a bottomthat is patterned and etched to define blade masking and having a spacerwafer fusion bonded to the device wafer.

FIG. 12B shows isolation trenches, vias, interconnect metal, mirrormetal, and trenches on the top of the device wafer.

FIG. 12C shows a window etched through the spacer wafer.

FIG. 12D shows blades formed using deep silicon etching and a base waferbonded to the spacer wafer using glass frit.

FIG. 12E shows a cross section after a release etch separates structuresand after a glass lid is bonded to the top of the wafer using fritglass.

FIGS. 13A through 13I show perspective views of the process flow offorming two parallel cantilevered beams.

DETAILED DESCRIPTION

The method and apparatus described herein may be used to providedecoupled rotation of structures about different pivot points. For oneembodiment, the apparatus may include one or more fixed blades mountedto a frame or substrate, one or more movable blades mounted to eachstructure to be moved, and flexures on which the structures aresuspended. Separate movable blades are provided for each degree offreedom.

When voltage is applied between the fixed and movable blades,electrostatic attraction generates a force attracting movable bladestoward blades that are fixed relative to the movable blades. Theelectrostatic attraction causes the structure to which the movable bladeis mounted to rotate about the flexures. The angle of rotation thatresults may be related to the size of the blades, the number of blades,the spacing between blades, the stiffness of the flexures, and themagnitude of the voltage difference applied to the blades.

Methods of fabricating a microelectromechanical apparatus are alsodescribed herein. The methods include the use of deep silicon etching toform blades.

FIGS. 2A and 2B illustrate one embodiment of an actuator. For oneembodiment, actuator 200 includes a stage 240 and a frame 235. FIG. 2Ashows the stage 240 parallel to the frame 235. FIG. 2B shows the stage240 tilted with respect to the frame 235. Stage 240 may have areflective element 245, such as a mirror, disposed on its top surface.Stage 240 is pivotally coupled to frame 235 using flexures 253 and 254on diametrically opposed sides of stage 240. Flexures 253 and 254suspend stage 240 in a cavity formed by frame 235 such that stage 240 isfree to pivot around a rotational axis formed by flexures 253 and 254.Stage 240 and frame 235 each have one or more blades (e.g., blades 220and 225, respectively) coupled to and extending from them. For example,blade 220 is coupled to stage 240 and blade 225 is coupled to frame 235.By applying a voltage difference between blades 220 and 225, stage 240may be pivoted.

Similarly, frame 235 may be pivotally coupled to an outer stationaryframe (not shown) using flexures 251 and 252 on diametrically opposedsides of frame 235. The outer frame may be a stationary frame or,alternatively, may be also be designed to move relative to yet anotherouter frame structure. Flexures 251 and 252 suspend frame 235 in acavity formed by the outer frame such that frame 235 is free to pivotaround a rotational axis formed by flexures 251 and 252. Flexures 251and 252 are orthogonal to flexures 253 and 254, thereby enabling areflective element coupled to stage 240 to be pivoted in two dimensions(e.g., rolled and pitched).

A blade is defined as a rigid object having any one of various shapes.For example, a blade may be a polyhedron as illustrated in FIGS. 2A and2B. Alternatively, blades may have other three dimensional polygonalshapes, for example, cubic and trapezoidal. A blade may either be asolid or hollow object.

Blade 220 extends in a direction perpendicular to the undersurface ofstage 240 and blade 225 extends in a direction perpendicular to theundersurface of frame 235. An electric potential applied between blades220 and 225 may cause an attraction between the blades. Because blade220 is coupled to stage 240, an attraction of blade 220 towards blade225 causes stage 240 to pivot about the rotational axis formed byflexures 253 and 254. For example, stage 240, and the correspondingblades coupled to the stage 240, may be pivoted such that the surface ofstage 240 lies at an angle relative to the surface of frame 235 as shownby the position illustrated in FIG. 2B. The operation of blades isdiscussed below in relation to FIGS. 3A and 3B.

FIG. 3A is a perspective view illustrating one embodiment of a bladeactuator. Blade actuator 311 includes a blade 312 that is part of astructure 322 to be actuated. For one embodiment, for example, structure322 may be a segment of stage 240 where blade 220 of FIGS. 2A and 2B isattached. Structure 322 may be constrained from vertical or lateralmotion but remains free to pivot on a torsional flexure 335. For oneembodiment, flexure 335 is rectangularly shaped. Alternatively, flexure335 can be any other shape that provides rotational compliance and thatcan be fabricated with integrated circuit fabrication techniques, forexample. The rotation of structure 322 allows for blade 312 to rotatewithin the X-Y plane (392, 391). By the design of flexure 335, themotion of blade 312 is constrained in the Z-direction (into/out of thepage) 393.

Actuator 311 also includes blade 313 that is part of structure 323. Forone embodiment, blade 313 corresponds to blade 225 of FIGS. 2A and 2Band blade 312 corresponds to blade 220 of FIGS. 2A and 2B. Blade 313can, for example, be attached to frame 235 and blade 312 can be attachedto stage 240. Given that blade 312 rotates within the X-Y plane (391,392) relative to blade 313, blade 312 is referred to as a movable bladeand blade 313 is referred to as a fixed blade.

Blades 312 and 313 may be configured as electrodes having electriccharges to generate an electrostatic field between them. Anelectrostatic field forms around any single object that is electricallycharged with respect to its environment. An object is negatively charged(−) if it has an excess of electrons to its surroundings. An object ispositively charged (+) if it is deficient in electrons with respect toits surroundings. Objects attract if their charges are of oppositepolarity (+/−) and repel if their charges are of the same polarity(+/+or −/−).

An electrostatic field also arises from a potential difference, orvoltage gradient, that exits when charge carriers, such as electrons,are stationary (hence the “static” in “electrostatic”). When two objects(e.g., blades 312 and 313) in each other's vicinity have differentelectric charges, an electrostatic field exists between them. As such,when a voltage is applied between blades 312 and 313, an attractiveforce is produced between them. The attractive force between blades 312and 313 is proportional to the square of the voltage potential betweenthem.

When there is no voltage potential between blades 312 and 313, thesurface 352 of structure 322 is substantially parallel with the surface353 of structure 323 and blade 312 is separated from blade 313 adistance 330 in X direction 392. The distance 330 can either positive ornegative—i.e., the blades 312 and 313 can either be overlapping ornonoverlapping. As a voltage potential is applied between blades 312 and313, the movable blade 312 is attracted toward fixed blade 313 andstructure 322 pivots about flexure 335. The greater the height 333 ofblades 312 and 313, the greater the torque that is generated onstructure 322. The generation of a greater torque decreases the amountof voltage required to pivot structure 322. Because structure 322 (towhich blade 312 is coupled) is constrained to pivot on rotational axis335, movable blade 312 moves in Y direction 391 and moves towards fixedblade 313 in X direction 392 until surface areas of blades 312 and 313overlap, as illustrated in FIG. 3B.

Because the blade 312 can rotate about an axis 335 which may be in theform of a torsional spring such as 254, it is convenient to view of theforce of attraction between blades 312 and 313 as being a torque. Thistorque that acts on blade 312 as a result of applying a voltagedifference between blades 312 and 313 is approximately proportional tothe height 333 of the blades squared and inversely proportional to thegap 332 as shown in the equation below. $\begin{matrix}{{torque} = {\frac{1}{2}\quad \frac{ɛ_{o}{height}^{2}{voltage}^{2}}{gap}}} & \text{(Equation~~1)}\end{matrix}$

As shown in FIG. 3C, for one embodiment, based on length 340 and height333, a portion 319 of blade 312 may no longer overlap blade 313 towardsthe end of the blade 312's stroke range. As the leading tip 309 of blade312 moves past the edge 308 of blade 313, the torque may taper off. Assuch, for a given height 333, the stroke range may be primarilydetermined by the length 340 of blades 312 and 313.

For one embodiment, blades 312 and 313 have a length 340 and a height333 each on the order of hundreds of microns and widths 331 on the orderof tens of microns. For one embodiment, for example, a structure may berotated an angle (θ) that may be greater than 20 degrees relative to thestructure's resting position.

For an alternative embodiment, blades 312 and 313 may have differentlengths, heights, and widths, which may also be different with respectto each other.

The overlap between blades 312 and 313 and the geometric shape of theleading edge of the blade are important factors with respect to forceprofile over the deflection angle.

Because movable blade 312 is constrained from motion in Z direction 393,the distance 332 between blades 312 and 313 remains substantiallyconstant along the stroke range of blade 312. As shown in equation 1 setforth above, the torque produced between blades 312 and 313 isproportional to 1/gap. Because the gap remains substantially constantalong the stroke range of blade 312, the torque also remainssubstantially constant for a given voltage. The gap 332 between theblades can be substantially smaller than the gaps used in the priorart—for example, gap 13 of FIGS. 1B and 1C is 150 microns. Gap 332 ofFIG. 3A is typically on the order of five to twenty microns. The netresult of having a small and constant gap is that high forces andtherefore high torques are produced over the entire stroke range ofblade 312. In this manner, a larger deflection angle of blade 312 may beachieved with a lower voltage than previously required with prioractuators. As an example, approximately ⅓ less voltage may be used tocontrol the actuation of stage 240 of FIGS. 2A and 2B. For oneembodiment, for example, the actuation voltage may be on the order of100 volts.

FIG. 3D is a top view of an alternative embodiment of an actuator wherean additional fixed blade 314 may be used to further increase theattraction force on movable blade 312 and, thereby, reduce the voltagenecessary for blade actuation.

As shown in FIG. 3E, an additional fixed blade 329 may also be placed onthe other side of movable blade 312 to rotate blade 312 in bothdirections. Alternatively, an additional movable blade 221 may be usedin conjunction with additional fixed blade 226, as illustrated in FIGS.2A and 2B. In this manner, there is one fixed blade and one movableblade for each direction of motion.

FIG. 4 illustrates an alternative embodiment of blade for an actuator.In one embodiment, either of blades 412 and 413 may be tapered alongtheir lengths (e.g., length 440 of blade 412). In this configuration,the effective separation 432 between blades 412 and 413 decreases as thesurface areas of the blades along their lengths overlap one another,thereby resulting in an increasing level of force with the increasingdeflection of blade 412. As discussed above, an increase in force meansthat a lower voltage is required to maintain the attraction betweenblades 412 and 413. In addition, the tapering of movable blade 412 mayimprove the off-axis (i.e., Z direction 393 of FIGS. 3A and 3B)instability of the blade. The edges of the blades may also be shaped tocontrol the initiation performance of blade 412 as it first starts tomove in X direction 492.

FIG. 5A is a top view showing one embodiment of a multiple stageactuator. For one embodiment, actuator 500 includes a central stage 501,a movable frame 502, and a stationary frame 514. Stationary frame 514forms a cavity in which stage 501 and movable frame 502 are disposed. Areflective element (e.g., a mirror) may be coupled to stage 501 andsuspended from movable frame 502 by a pair of flexures 503 a and 503 b.The reflective element may be used to redirect a light beam along anoptical path different from the optical path of the received light beam.An actuator 500 that includes a mirror on stage 501 is also referred toas a mirror cell or a MEM actuator with a mirror.

For one embodiment, the rotation of stage 501 is independent of therotation of movable frame 502. Actuator 500 thus allows decoupledmotion. For example, stage 501 can rotate with respect to frame 502while frame 502 remains parallel and stationary with respect to frame514. In addition, movable frame 502 can rotate with respect tostationary frame 514 while stage 501 remains parallel (and stationary)with respect to movable frame 502. Furthermore, stage 501 and movableframe 502 can, for example, both rotate concurrently yet independentlyof each other. Thus, for example, stage 501, movable frame 502, andstationary frame 514 can concurrently be non-parallel and decoupled withrespect to each other during actuation.

Flexures 503 a and 503 b are coupled to movable frame 502 via end bars516 a and 516 b, respectively. End bars 516 a and 516 b are, in turn,attached to the main body of movable frame 502 using multiple supportmembers 505. Support members 505 are silicon dioxide beams providing atensioning force. The support members 505 provide a tensioning force byexpanding a different amount than the material system used in fabricateframe 502, stage 501, end bars 516 a and 516 b, and stationary frame514. The concept is to place material systems of differing expansioninto the movable frame 502 in order to put the flexures 503 a, 503 b,504 a, and 504 b into tension. In particular, the expansion provided bymembers 505 acting against frame 502 and end bars 516 a and 516 b causesa tensioning force on each of flexures 503 a, 503 b, 504 a, and 504 b.Support members 505 serve to apply a tension force in order to minimizethe potential for positional distortions due to buckling of the flexuresunder compressive forces. Generally, if flexures 503 a, 503 b, 504 a,and 504 b are under too great a compressive force, flexures 503 a, 503b, 504 a, and 504 b may buckle. As such, support members 505 may becoupled between the main body of movable frame 502 and end bars 516 aand 516 b at a non-perpendicular angle 509 in order to pull on flexures503 a and 503 b to place them in tension. Because flexures 504 a and 504b are perpendicular to flexures 503 a and 503 b, the non-perpendicularangle 509 of attachment of support members 505 causes a pull on the mainbody of movable frame 502 and, thereby, a pull on and a tensioning offlexures 504 a and 504 b.

For one embodiment, for example, support members 505 may be coupledbetween the main body of movable frame 502 and end bars 516 a and 516 bat approximately a 45 degree angle. In an alternative embodiment,support members 505 may be coupled between the main body of movableframe 502 and end bars 516 a and 516 b at an angle less than or greaterthan 45 degrees.

Flexures 503 a and 503 b allow central stage 501 to pivot. Flexures 503a and 503 b provide some torsional resistance proportional to therotation angle, but substantially less resistance than all otherdirections. In other words, there is substantial resistance to undesiredtwisting movement of central stage 501 in other directions (e.g.,side-to-side, or around an axis perpendicular to the surface of centralstage 501). Flexures 503 a and 503 b extend into slots 517 a and 517 b,respectively, formed into central stage 501 in order to providesufficient length to the flexures for appropriate flexibility andtorsion resistance. In one embodiment, for example, flexures 503 a and503 b may have a length of approximately 100 microns, a height ofapproximately 10 microns, and a width of approximately 1 micron,resulting in a 10:1 aspect ratio. Such an aspect ratio may provide forgreater compliance in the direction of desired motion and stiffness inthe undesired directions. In an alternative embodiment, other lengths,heights, widths, and aspect ratios may be used.

Similarly, flexures 504 a and 504 b enable movable frame 502 to pivotwhile providing resistance to undesired twisting movement of movableframe 502 in other directions (e.g., side-to-side, or around an axisperpendicular to the surface of movable frame 502). Flexures 504 a and504 b extend into slots 518 a and 518 b, respectively, formed intomovable frame 502 and stationary frame 514 in order to providesufficient length to the flexures for appropriate flexibility andtorsion resistance.

For one embodiment, one or more of flexures 503 a, 503 b, 504 a, and 504b may comprise a pair of torsion beams. The use of multiple torsionbeams may provide for increased resistance to undesired twistingmovement of a frame or stage, as compared to a single beam flexure. Apair of torsion beams may have various configurations. For example, apair of torsion beams may have the configuration of torsion beams 524and 525 as illustrated in the close-up view of FIG. 5B. Torsion beams524 and 525 may be non-parallel beams whose ends near movable frame 502are substantially parallel and spaced apart by a gap 528. Gap 528between torsion beams 524 and 525 reduces along the length of the beamssuch that the ends of the beams near fixed frame 514 are closer togetherthan the ends of the beams near movable frame 502. The angling oftorsion beams 524 and 525 relative to each other may aid flexure 504 ato resist unstable twisting modes. In an alternative embodiment, torsionbeams 524 and 525 may be configured such that their ends near fixedframe 514 are farther apart than their ends near movable frame 502. Inyet another embodiment, torsion beams 524 and 525 may be substantiallyparallel to each other such that gap 528 is substantially uniform alongthe length of the beams.

Alternatively, as shown in FIG. 5C, flexure 503 b may comprises a pairof torsion beams as shown by torsion beams 526 and 527. Torsion beams526 and 527 are substantially parallel beams spaced apart by gap 529.Gap 529 between torsion beams 526 and 527 remains substantially constantalong the length of beams 526 and 527. The parallel torsion beams 526and 527 may operate to enhance the mechanical stability of central stage501. In an alternative embodiment, torsion beams 526 and 527 may beconfigured such that their ends near central stage 501 are closertogether than their ends near end bar 516 b. For yet another embodiment,torsion beams 526 and 527 may be configured such that their ends nearend bar 516 b are closer together than their ends near central stage501.

FIG. 6 illustrates one embodiment of the underside of the multiple stageactuator 500 of FIG. 5A. For the embodiment illustrated in FIG. 6, ateach end of a stage or frame, actuator 500 uses a single movable bladewith two corresponding fixed blades as an actuation mechanism structureto enable rotation. Actuator 500 uses two such actuation mechanismstructures per stage and two such actuation mechanism structures perframe.

In the illustrated embodiment, blade 612 is coupled to stage 501 andblades 613 a and 613 b are coupled to frame 502 on opposite ends ofblade 612. Stage 501 is pivotally coupled to frame 502 such that blade612 is configured to move relative to blades 613 a and 613 b. When apotential difference is applied between blade 612 and one of blades 613a and 613 b, an attraction is generated between the blades causing stage501 to pivot. For example, blade 612 may be held at a ground potentialwhile an active voltage is applied to either of blades 613 a and 613 b.The application of an active voltage to blade 613 a will attract blade612 (as discussed above in relation to FIGS. 3A and 3B), thereby causingstage 501 to rotate in a corresponding direction. Similarly, theapplication of an active voltage to blade 613 b will attract blade 612and cause stage 501 to rotate in an opposite direction to that resultingfrom the attraction to blade 613 a.

Blade 622 is coupled on the opposite end of stage 501, with blades 623 aand 623 b coupled to frame 502 on opposite ends of blade 622. Blade 622moves relative to blades 623 a and 623 b. In order to provide thedesired motion of stage 501 and to resist unwanted rotations, actuationvoltages are applied concurrently with respect to blades 612 and 622.When the potential difference is applied between blade 622 and one ofblades 623 a and 623 b, an attraction is generated between the bladesresulting in the rotation of stage 501 in a manner similar to thatdiscussed above. The use of actuation mechanisms in tandem on each endof stage 501 minimizes undesired twisting of the stage 501 to providefor more uniform rotation.

A similar actuation mechanism structure may be used for rotation offrame 502. For example, blade 611 is coupled to movable frame 502 andblades 610 a and 610 b are coupled to stationary frame 514 on oppositeends of blade 611. Frame 502 is pivotally coupled to frame 514, asdiscussed above, such that blade 611 is configured to move relative toblades 610 a and 610 b. When a potential difference is applied betweenblade 611 and one of blades 610 a and 610 b, an attraction is generatedbetween the blades causing frame 502 to pivot in a manner similar tothat discussed above in relation to stage 501.

Blade 621 is coupled on the opposite end of frame 502, with blades 620 aand 620 b coupled to frame 514 on opposite ends of blade 621. Blade 621moves relative to blades 620 a and 610 b. When the potential differenceis applied between blade 621 and one of blades 620 a and 620 b, anattraction is generated between the blades facilitating the rotation offrame 502. The use of actuation mechanisms in tandem on each end offrame 502 minimizes undesired twisting of the frame to provide for moreuniform rotation.

Alternatively, a stage or frame may only have an actuation mechanismstructure on only a single end. For another embodiment, actuator 500 mayhave other actuation mechanism structures as discussed above in relationto FIGS. 2A to 4.

For one embodiment, additional elongated members (e.g., elongated member615) may be coupled to the undersurface of stage 501 to stiffen stage501 and minimize top surface distortions. In addition, blades 615 onstage 501 may be used to remove etch depth variations across the device.Elongated member 615 may be constructed similar to that of bladesdiscussed above in relation to FIGS. 2A and 2B.

Because the actuation mechanism of actuator 500 is located entirelybeneath the stage to be rotated, none of the top surface areas of stage501 need be taken up by the actuation mechanism.

For one embodiment, actuator 500 may be fabricated on a wafer levelusing semiconductor fabrication techniques, as discussed below. For suchan embodiment, frame 514 may be formed from a substrate, for example,constructed from silicon. Where all blades are directly driven bydifferent control voltages, actuator 500 may use four voltages, plus aground, for the configuration illustrated in FIG. 6. With thisarrangement, the number of conductive paths on a substrate quicklybecomes very large as multiple actuators are combined to form an array,as illustrated in FIG. 7A. The low voltages required by the bladeactuators discussed above may allow for control circuitry to befabricated into the substrate so that only control signals need berouted, rather than separate lines for each blade. This results in asignificant reduction in lead count. Lower voltages may also reduce thenecessity for spacing between leads to avoid arcing and cross-talk

For one embodiment, transistors may be used to address mirrored stagesindividually, for example, using a row-column addressing scheme. Thismay significantly reduce the number of metal traces (e.g., trace 709)necessary to operate array 700. Interconnect metallization on the topsurface of actuator array 700 may be used to route voltages overflexures to different blades (not shown) on the underside of array 700,as illustrated in FIG. 7B. In one embodiment, metallization may be usedto form mirrors on the respective actuator stages resulting in aplurality of mirror cells.

FIG. 7B shows an example of how electrical contact can be made to theblade actuators. For example, flexure 712 is a movable frame flexuresimilar to torsion beam 525. Along the top surface of flexure 712 is ametal layer 713 that eventually runs over a portion of movable frame502. This metal layer 713 runs over isolation joint 717 and connects toan isolated region 718 of movable frame 502 at via 714. Under thisisolated region 718 is a blade used to tilt the central stage 501. Asimilar connection is made on the other side of the frame 502 at via715.

Flexure 712 also is made primarily of highly doped silicon. This siliconwithin flexure 712 conducts electricity between the fixed stage 514 andthe portion of the movable frame generally indicated by 716. Under themovable frame generally indicated by 716 is a movable frame blade usedto tilt the movable frame. Other alternative electrical routing schemesare possible by including additional isolation joints or additionaltorsional beams.

Isolation segments (e.g., isolation segments 706 and 707) may be used toseparate potentials for different sections of the substrate. Isolationsegments 706 and 707 are electrical barriers made of silicon dioxide (adielectric) that reside midway within the structural silicon beams 580shown in FIGS. 7B and 5A. Each of beams 580 includes isolation segmentsthat serve to electrically isolate sections of the frame 502 from oneanother. Each electrical isolation segment extends beyond the width anddepth of respective structural beams 580 in order to completely breakany potential conduction path. Support members 505 also provide suchelectrical isolation given that they are comprised solely of silicondioxide, which is a dielectric. Such electrical isolation is necessaryto allow separate electrical potentials to be applied to respectiveblades in order to create potential voltage differences between bladesto trigger actuation. Without such isolation segments, continuousconduction paths within the silicon would short the actuation potentialsbetween the blades.

For one embodiment, electrodes for each mirror may be routed on the topsurface of array 700 of FIG. 7A using standard techniques known in theart. In an alternative embodiment, electrodes may be routed directly tothe backside of a wafer using through-wafer vias to increase packingdensity.

A number of techniques may be used to fabricate mirror cell 500 shown inFIG. 8. The techniques discussed with respect to FIGS. 9A-9K areassociated with the view provided by cross-section line 801 shown inFIG. 8. The fabrication methods of embodiments of the invention resultin a mirror platform suspended by cantilevered silicon beams. Electricalisolation between sections of the mirror or between different blades isachieved through the use of integral isolation segments, which serve tomechanically connect but electrically isolate separate elements of themirror.

A major design parameter for the mirror actuator is the depth of theblades, measured perpendicular to the axis of rotation. Increasing theblade depth results in increased force, but requires more swing space torotate through high angles. Shallower blades more easily accommodatehigher deflections but usually require a greater number of blades inorder to achieve the same force. Therefore, it is advantageous to haveseveral blade depths available to the designer. Different blade depthsrequire multiple approaches to the fabrication process, which aredescribed herein.

One embodiment of the invention uses a single device wafer and theassociated method is set forth with reference to FIGS. 9A-9K. FIG. 9Ashows a silicon wafer 901 that is chosen to be in the thickness range of300-600 micrometers (um). The silicon wafer 901 has a topside (or deviceside or simply a top) 906 and a backside or bottom 907.

FIGS. 9B-9E illustrate an upper lefthand portion 1102 of wafer 901 incross section to show a process for fabrication of isolation trenches1120 on the device side 906 of wafer 901. The trenches 1120 are filledwith a dielectric material, which for one embodiment is silicon dioxide.The trenches 1120 so filled provide the electrical isolation betweenblades after the mirror is released. A dielectric layer 1103 alsoremains on the surface of the wafer 901 and is planarized after the fillprocess to ease subsequent lithographic patterning and eliminate surfacediscontinuities.

Referring to FIG. 9B, a silicon wafer 1102 is provided with a dielectriclayer 1104, which for one embodiment is silicon dioxide (i.e., an oxidelayer). The silicon wafer can be of arbitrary doping, resistivity, andcrystal orientation, because the process depends only on reactive ionetching to carve and form the structures. The layer 1104 serves thefunction of protecting the silicon surface of the wafer during anisolation trench etch to follow, and thus represents a masking layeronly. This masking layer can be formed from any number of techniques,including thermal oxidation of silicon or chemical vapor deposition(CVD). The typical thickness of the masking layer 1104 is 0.5-1.0 um. Aphotoresist 1106 is then spun onto the wafer and exposed and developedusing standard photolithography techniques to define the isolationtrench pattern 1108. Reactive ion etching is used to transfer thephotoresist pattern to the mask layer 1104, as at 1110, exposing thesilicon surface 1112. Typically, the silicon dioxide mask is etched inFreon gas mixture, for example CHF₃ or CF₄. High etch rates for silicondioxide etching are achieved using a high density plasma reactor, suchas an inductively coupled plasma (“ICP”) chamber. These ICP chambers usea high power rf source to sustain the high density plasma and a lowerpower rf bias on the wafer to achieve high etch rates at low ionenergies. Oxide etch rates of 200 nm/min and selectivities tophotoresist greater than 1:1 are common for this hardware configuration.

As illustrated in FIG. 9C, an isolation trench 1114 is next formed inthe wafer 102 by deep reactive ion etching of silicon using high etchrate, high selectivity etching. The trench is commonly etched in a highdensity plasma using a sulfur hexaflouride (SF₆) gas mixture asdescribed in U.S. Pat. No. 5,501,893. Preferably, the etch is controlledso that the trench profile is reentrant, or tapered, with the top 1116of the trench being narrower than the bottom 1118 of the trench. Thistapering ensures that good electrical isolation is achieved insubsequent processing. Profile tapering can be achieved in reactive ionetching by tuning the degree of passivation, or by varying theparameters (power, gas flows, pressure) of the discharge during thecourse of the etch. Because the trench is to be filled with dielectric,the opening at the top 1116 of the trench is chosen to be less than 2 umin width. The trench depth is typically in the range 10-50 um. A commonprocedure for etching the trench is to alternate etch steps (SF₆ andargon mixture) with passivation steps (Freon with argon) in an ICPplasma to achieve etch rates in excess of 2 um/min at high selectivelyto photoresist (>50:1) and oxide (>100:1). The power and time of theetch cycles are increased as the trench deepens to achieve the taperedprofile. Although the trench geometry is preferably reentrant, arbitrarytrench profiles can be accommodated with adjustments in microstructureprocessing. Good isolation results can be achieved with any of a numberof known trench etch chemistries. After the silicon trench is etched,the photoresist layer 1106 is removed with wet chemistry or dry ashingtechniques, and the masking layer 1104 is removed with a reactive ionetch (“RIE”) or buffered hydrofluoric acid.

Referring to FIG. 9D, the isolation trench 1114 is then filled with aninsulating dielectric material, typically silicon dioxide. The fillingprocedure results in the mostly solid isolation segment 1120 in thetrench 1114, and serves to deposit a layer 1122 of dielectric materialon the top surface 1112 of the silicon wafer and dielectric layers onthe sidewall 1124 and bottom 1126 of the trench. The thickness of thedeposited layer is usually in excess of 1 um. This fill can beaccomplished with chemical vapor deposition (“CVD”) techniques orpreferably with oxidation of silicon at high temperatures. In thermaloxidation, the wafer is exposed to an oxygen rich environment attemperatures from 900-1150° C. This oxidation process consumes siliconsurfaces to form silicon dioxide. The resulting volumetric expansionfrom this process causes the sidewalls of the trenches to encroach uponeach other, eventually closing the trench opening. In a CVD fill, somedielectric is deposited on the walls but filling also occurs fromdeposition on the bottom of the trench. CVD dielectric fill of trencheshas been demonstrated with TEOS or silane mixtures in plasma enhancedCVD chambers and low pressure CVD furnace tubes.

During a trench fill, it is common for most trench profiles to beincompletely filled, causing an interface 1128 and a void 1130 to beformed in the trench. A local concentration of stress in the void cancause electrical and mechanical malfunction for some devices, but isgenerally unimportant for micromechanical devices due to the enclosedgeometry of the isolation segment 1120. The interface 1128 and void 1130can be eliminated by shaping the trench to be wider at the trenchopening 1116 than the trench bottom. However, good electrical isolationwould then require additional tapering of the microstructure trench etchin the later steps. Another artifact of the trench filling is anindentation 1132 that is created in the surface of the dielectric 1134centered over the isolation segment 1120. This indentation isunavoidable in most trench filling processes, and can be as deep as 0.5um, depending on the thickness of the deposition.

To remove the indentation 1132, the surface is planarized to form a flatsurface 1136, as illustrated in FIG. 9E, for subsequent lithographic anddeposition steps. Planarization is performed by depositing a viscousmaterial, which can be photoresist, spin-on glass, or polymide, andflowing the material to fill the indentation 1132 to a smooth finish.During etchback, which is the second step of planarization, the surface1136 is etched uniformly, including the filled indentation. Therefore,by removing part of the surface oxide 1122, the indentation 1132 isremoved to create a uniform thickness layer 1138. For example, if theoriginal dielectric layer 1122 is 2 um, then planarization to remove theindentation 1132 leaves a dielectric layer 1138 having a final thicknessof less than 1 um. The surface 1136 of wafer is free from imperfectionand is ready for further lithography and deposition.

FIG. 9F shows silicon wafer 901 with dielectric layer 903 and isolationtrenches 1120. After the isolation trenches 1120 are fabricated,standard front-to-back alignment is used to lithographically pattern themasking layer for the blades on the backside 907 of the wafer. The bladepattern 904 is exposed and etched into a dielectric masking layer 905.The masking layer is typically comprised of a combination of thermallygrown silicon oxide and oxide deposited by chemical vapor deposition.The lithography pattern is transferred in the masking layer by reactiveion etching, yet the silicon blade etching is not completed until laterin the process. Without the blades etched, the wafer is easily processedthrough the remaining device layers. The backside blade pattern 904 istypically aligned to the topside isolation trenches 1120 to withinseveral microns.

Metallization on the topside 906 of the wafer then proceeds as in FIG.9G. In order to make contact to the underlying silicon 908 vias 909 arepatterned and etched into the dielectric layer 903 using standardlithography and reactive ion etching. After the vias are etched,metalization 910 is deposited and patterned to form an interconnect 911and a contact 912 to the silicon 908 through the via 909. For oneembodiment, the metal is aluminum and is patterned using wet etchingtechniques. In mirror arrays with high interconnect densities, it isadvantageous to pattern the metal using dry etching or evaporated metallift-off techniques to achieve finer linewidths. The metal layer 910 isused to provide bond pads and interconnects, which connect electricalsignals from control circuitry to each mirror to control mirroractuation.

Deposition of a second metal layer 913 provides a reflective mirrorsurface. This metal is tuned to provide high mirror reflectivities atthe optical wavelengths of interest, and is typically evaporated andpatterned using lift-off techniques to allow a broader choice ofmetallizations. For one embodiment, the metallization is comprised of500 nm of aluminum. However, additional metal stacks such as Cr/Pt/Aumay be used to increase reflectivities in the wavelength bands common tofiber optics. Because the metals are deposited under stress and willaffect the eventual mirror flatness, it is advantageous to reduce thethickness of the dielectric 914 in the region of the mirror. This can beaccomplished through the use of dry etching of the underlying dielectricprior to evaporation.

In FIG. 9H, the topside processing is completed. First, a passivationdielectric 915 on the metal surfaces 911 and 913 may be applied toprotect the metallization during subsequent processing. The passivationis removed in the region of the bonding pads. Second, the mirrorstructure including frame, mirror, and supports are defined usingmultiple etches that define trenches 916 separating the structuralelements. The etches are self-aligned and proceed through the variousmetal 910, dielectric 903, and silicon 908 layers. A further blanketdeposition is applied to the topside which passivates the sidewalls ofthe trenches 916 and prepares the topside for mechanical release.

As shown in FIG. 9I, backside silicon etching transfers the bladepattern 904 into the substrate 908 to obtain the blades 918. The etchingis performed using deep silicon etching at high selectivity to oxideusing the techniques reported in U.S. Pat. No. 5,501,893 and nowcommonly used in the industry. The deep silicon etching achieves nearvertical profiles in the blades 918, which can be nominally 5-20 um wideand in excess of 300 um deep. The etch is timed so that the etch front919 approaches or just reaches the bottom of the isolation joints 1120or the structure trenches 916, yet not to not punch through to thetopside surface of the wafer 906. All blades 918 are etchedsimultaneously across the mirror element and across the mirror array.

Referring to FIG. 9J, because the device wafer 920 is now prepared formicrostructure release, the device wafer 920 becomes more susceptible toyield loss due to handling shock or air currents. In order facilitatehandling and aid in hermetically sealing the mirror array, a base wafer921 is bonded to the device wafer 920 to protect the blades afterrelease. For one embodiment, the bonding is accomplished through the useof a frit glass material 922 that is heated to its flow temperature andthen cooled. In this manner, a 400 degree centigrade temperature bondproduces a hermetic seal 923 to surround the entire mirror array. Theseparation between the device wafer 920 and the base wafer 921 using thefrit glass 922 allows the blades to swing through high rotation angleswithout impedance. Typically, the standoff required is greater than 25um.

Final structure release is accomplished on the wafer topside in FIG. 9Kusing dry etching, which punctures through the trenches 916 to suspendthe movable elements of the mirror 924 and the frame 925. In addition,the release etch promotes electrical isolation by separating, forexample, the silicon of the frame 927 from the silicon of surroundingmembers 928 and 920. The vias 909 serve to connect the regions ofsilicon to the metal interconnects 911. To completely seal the mirrorsfrom the outside environment, a lid wafer 930 is bonded to the devicewafer 920, preferably through the frit glass seal 931. The lid wafer 930is typically glass to allow incoming light to be transmitted with lowloss in the mirror cavity 932, reflect off of the mirror surface 913,and transmit out of the mirror cavity.

FIGS. 10A-10E illustrate an alternative embodiment of the inventionassociated with a cross section of an alternative mirror cell (notshown) that cuts across four blades and three suspended sections of themirror cell. For the embodiment of the invention shown in FIGS. 10A-10E,a bond and polish sequence is used to tune the depth of the blades to avalue substantially less than the thickness of a normal wafer. Becausethinner wafers are fragile and subject to significant handling loss, thebase wafer is used early in the process to provide handling support. InFIG. 10A, the blades 1201 are patterned and etched using deep siliconetching techniques into the device wafer 1202 at the beginning of theprocess. The depth of the blade trench 1206 is tunable and depends ondesign, swing, and actuator deflection requirements. The blade depth maybe 200 um, for example. A base wafer 1203 is then fusion bonded to thedevice wafer at the interface 1204. The fusion bonding process directlybonds silicon to silicon or silicon oxide and requires a hightemperature anneal to form a strong bond. A recess 1207 is etched intothe base wafer 1203 to provide the space necessary for the blades torotate.

To proceed with topside processing, the device wafer 1202 is thenpolished down to establish a new topside surface 1205. This polishingstep may remove several hundred microns of material. After thepolishing, topside processing is performed. In FIG. 10B, isolationtrenches 1208, vias 1210, metal interconnects 1205, and mirrormetalization 1211 are fabricated in accordance with the sequence inFIGS. 9A-9H.

Alignment must be maintained between the device topside features and theblades. Several techniques are available to accomplish such alignment.For example, infared illumination passing through the wafer can be usedto identify the location of the buried structures, such as blades. Foranother method, alignment marks can be placed on the backside of basewafer 1203. Those alignment marks are aligned with respect to theblades. Such marks require that the base wafer 1203 be carefully alignedwith respect to wafer 1202 during the fusion bonding process. After thefusion bonding process, the topside features are then aligned withrespect to the alignment marks on the base wafer 1203. Any number ofthese schemes may be used to ensure that the topside features align withrespect to blades to within several microns.

In FIG. 10C, structure trenches 1212 are etched into device wafer 1202,and release etching in FIG. 10D suspends the mirror 1213 and frame 1214and frees the micromechanical mirror for motion. The entire devicesequence can be performed because the bond interface 1204 remainsunaffected by temperature cycling and unit processing after the bondanneal. Finally, in FIG. 10E, a glass lid 1215 is bonded to the devicewafer 1202 using frit glass 1216 to hermetically seal the element fromthe environment.

Other substrates such as silicon-on-insulator can be used with onlyslight modifications to the process. In FIG. 11, the device wafer usinga silicon-on-insulator (SOI) substrate 1301 is shown. A thin siliconlayer 1302 is separated from the blade layer 1303 by buried oxide layer1304. Typically, the silicon layer is of the order of 5-20 um thick. Theblade layer is typically 300-600 um thick. The oxide layer 1304 isolatesthe blades 1305 from the blades 1306 without the need for the isolationsegments or trenches described with respect to FIGS. 9A-9K and 10A-10E.Vias 1307 connect through the oxide layer 1308 and connect to the blades1305 and are isolated from the silicon layer 1310 through passivation ofthe via sidewalls. The requirements for metallization, structuredefinition, and microstructure release remain fundamentally the same asfor the embodiment discussed with respect to FIGS. 9A-9K.

Because deep silicon etching is generally highly selective to siliconoxide, the buried layer 1304 provides an etch stop for the blade etchand also the structure etch that defines the trenches 1311. Releaseetching may be accomplished by etching through the oxide layer 1304 orby undercut methods.

The SOI method of FIG. 11 replaces the need for isolation trench etchand fill, but does not significantly impact the other aspects of theprocess flow. Thus, the fabrication techniques described with respect toFIGS. 9A-9K, 10A-10E, and 12A-12E can be used with the SOI substrate1301 shown in FIG. 11, but without the need for respective isolationtrenches.

Another improvement in the bonded wafer processes is shown in FIGS.12A-12E. For this approach, the masking dielectric layer is patterned inthe outline of the blades before fusion bonding, yet the bladesthemselves are not etched until later in the process. This enables thewafer stack to proceed through the polishing and trench isolationprocesses without compromising wafer fragility or introducingproblematic membrane structures. In FIG. 12A, the backside dielectric1402 of the device wafer 1401 is patterned and etched to define theblade masking 1406. The etch is not completed to the backside siliconsurface 1407. Instead, a small amount of dielectric 1403 is left.Typically, the thickness of the dielectric 1403 is 500 nanometers (nm),and the total thickness of masking layer 1402 is 3 um. The device wafer1401 is then fusion bonded to a spacer wafer 1404, bonding only at theblade patterns 1406. Sealed cavities 1408 remain at the bond interfaceafter the bond anneal. Next, the device wafer is polished to interface1405 to match the desired blade depth.

In FIG. 12B, the device wafer 1401 is processed in the manner of FIGS.9A-9H to obtain filled isolation trenches 1409, vias 1410, interconnectmetal 1411, mirror metal 1412, and trenches 1413. Alignment techniquessuch as those described with respect to FIG. 10B can be used to alignsuch topside features to the blade patterns 1406. In FIG. 12C, a windowor opening 1414 is patterned and etched through the silicon of spacerwafer 1404, which exposes the blade pattern 1406 in dielectric layer1402. The silicon etch is highly selective and will stop on the bladepattern 1406 and the remaining dielectric mask 1403.

In FIG. 12D, the partially etched dielectric 1403 is removed in blanketetching, and blades 1415 are etched to desired depth. A base wafer 1416bonded using glass frit 1417 ensures that the blades 1415 are protectedfrom further damage. A cavity 1418 houses the blades, which are recessedfrom the bottom plane 1419 of the spacer wafer 1404.

Finally, in FIG. 12E, the mirror structure is released by extending thetrenches 1420 through the remaining silicon membranes. A glass lid 1421bonded through frit glass 1422 completes the processing. The advantageof the recessed blade approach of FIGS. 12A-12E is that the bladeetching is withheld until later in the process, ensuring that the deviceand spacer wafer stack is mechanically robust during polishing and themajority of the processing, and hence planarity of the top surface ofthe device wafer is ensured during all lithographic steps.

FIGS. 13A-13I show fabrication details for forming the structural beamelements 580 and flexures 503 a, 503 b, 504 a, and 504 b of mirror cell500 shown in FIG. 5A. In particular, FIGS. 13A-13I illustrate, inperspective view, a process for forming two parallel cantilevered beams,each including an isolation segment. The parallel cantilevered beams canbe structural elements or flexures that may or may not include isolationsegments.

Referring to FIG. 13A, the process begins with a silicon wafer 6202 thathas a dielectric masking layer 6204, which for one embodiment is silicondioxide, and photoresist layer 6206. It is possible to begin the processwithout the dielectric layer and rely only on photoresist to mask theisolation trench etch. The photoresist is exposed and developed tocreate two isolation trench openings 6208 and 6210. This pattern istransferred to the dielectric using RIE, exposing the surface of thesilicon substrate 6202. Isolation trenches are then etched into thesubstrate silicon using silicon RIE, with the depths and profilesdescribed in detail in the description of FIGS. 9B-9E. The resist layer6206 and the dielectric layer 6204 are stripped in preparation fortrench filling.

In FIG. 13B, the isolation trenches are filled using thermal oxidationor CVD techniques to create two isolation segments. The filling processresults in a thick dielectric layer 6212 and indentations 6214 in thesurface of the dielectric 6216, producing two solid isolation segmentsthat are to be incorporated within the micromechanical structure. Toremove the indentations, the surface is planarized using a depositionand etchback process. FIG. 13C shows the results of the planarization,which has removed most or all of the surface indentation 6214, leavingminimal features 6220 in the areas where the isolation trenches exist.The dielectric 6218 thickness will remain as a masking material and aninsulating material for the final microstructure, and must thereforeretain good electrical and mechanical qualities. The thickness ofdielectric layer 6218 is preferably 0.5-1.0 um.

The next photolithographic step is illustrated in FIG. 13D, where a viapattern 6222 is exposed and developed in the photoresist layer 6224 by anormal lithography process. The resist pattern is transferred throughthe dielectric layer 6212 by reactive ion etching to reveal the siliconsurface 6226 in the region of the via. Alternatively, the revealedsilicon surface 6226 may remain protected by a thin sacrificial layer ofdielectric 6212 in order to minimize surface damage during implantation.The wafer 6202 is implanted with dopants in the region of the via 6222,so as to provide a high conductivity region in the substrate 6202. Ahigh temperature anneal activates the implant and prepares the wafer formetalization.

Metalization and coarse patterning of the metal is illustrated in FIG.13E. A metal layer 6228, which for one embodiment is sputtered aluminum,is deposited onto the top surface of dielectric layer 6212, whichinsulates the metal from the silicon, except in regions where a via 6226has been opened. The metal contacts the silicon in the via to form anohmic contact. Because the metal is sputtered on a mostly continuousdielectric surface, the resulting metal layer can be patterned easily,using lithographic methods. To do this, a layer of photoresist (notshown) is exposed and developed, and the pattern is transferred to themetal with wet chemical etching or RIE. Because the feature sizes aregenerally greater than 5 um for this coarse patterning step, theselithography and etching steps are generally non-critical. The purpose ofthis coarse metal patterning step is to define multiple interconnectsand pads for the microstructure to be formed in the wafer. Thus, forexample, pads 6230 and 6232 are aligned with the eventual placement ofmicrostructure beam elements and are separated by a gap 6234. Metal isalso removed in region 6236 to break the conduction path on one of theeventual beam elements.

The final lithography layer, which is used to produce themicromechanical structure, is exposed and developed according to theillustration in FIG. 13F. The photoresist pattern (not shown) istransferred to the metal layer 6228 and to the dielectric layer 6212using RIE techniques and defines an opening 6238 in the metal anddielectric layers in which beam elements 6240 and 6242 are placed. Thisopening serves as a mask for subsequent trench etching steps. Thelithography also defines in the metal layer 6228 two metal interconnects6244 and 6246 that attach to pads 6230 and 6232, respectively. The viathrough the dielectric layer 6212 defined in FIG. 13D is apparent at thelocation of the metal-silicon contact at 6248. The metal interconnect6244 is terminated at location 6250, a result of the coarse metalpatterning step.

The pattern transfer process etches the metal and dielectric to exposethe silicon surface 6252 and isolation segments 6254 and 6256 in themask opening 6238. The photoresist layer may remain or be removed forthe deep silicon trench etch illustrated in FIG. 13G, which defines adeep trench 6258 surrounding silicon mesas or islands 6260 and 6262. Thetrench etch is carried out to a depth less than the depth of theisolation segments 6254 and 6256, which are exposed during the etch, asillustrated. The isolation segments are positioned by the lithographyprocess so that they completely intersect and are perpendicular to themesas 6260 and 6262. The anisotropic nature of the etch, coupled withthe reentrant geometry of the segments themselves, ensures that nosilicon filaments surround the exposed surface 6264 of the segments 6254and 6256, for such filaments eventually would provide a current path tothe substrate 6202. The single mask opening 6238 forces the metalinterconnects 6244 and 6246 to be self-aligned with the dielectriclayers 6266 and 6266 and the respective mesas 6260 and 6262. The etchprocess used for one embodiment of the invention is the Bosch processdescribed in U.S. Pat. No. 5,501,893, which etches silicon selectivelyto the metal layer 6228 and the dielectric layer 6212 so that nodegradation of the layers occurs during the structure trench etch.

In FIG. 13H, the microstructure is prepared for undercut and release ofthe beams according to U.S. Pat. No. 5,719,073 by a sidewall passivationscheme. A dielectric, which for one embodiment is silicon dioxide, isdeposited using CVD techniques and forms a thin film on all surfaces.The thickness of the deposited film is less than 500 nm, and must bedeposited at a temperature that will not harm the metal layer 6228. Forone embodiment, the film that will form the sidewall passivationdielectric is deposited using plasma enhanced CVD (“PECVD”) or highdensity plasma CVD (“HDPCVD”) techniques and conformally coats allexposed surfaces. After the deposition, a blanket anisotropic RIE etchremoves the film from the floor 6270 of the trench 6258 and from allother horizontal surfaces, such as the top surface 6272 of the metallayer 6228. Due to the anisotropic nature of the etch, sidewall films6274 of the mesas remain intact to provide sidewall passivation whichprotects the silicon mesas 6260 and 6262 from the isotropic siliconrelease etch processes.

FIG. 13I illustrates a released microstructure after a release etchsequence that follows sidewall passivation. Often, the release etch iscomprised of two separate etches—namely, a trench extension that exposesa larger silicon surface area and an isotropic release etch thatundercuts the silicon mesas to form released beams 6276 and 6278. Thetrench extension is similar to the structure etch of FIG. 13G, anddeepens the trench 6258 to expose silicon below the sidewall film 6274.This is followed by an isotropic release etch, which can be performed ina high density etch chamber in a mixture of SF₆ and Argon. The releaseetch is timed so that beams 6276 and 6278 are completely undercut andsuspended over the silicon floor 6280, while wider features such as wall6282 remain fixed to the substrate. The isolation segments 6254 and 6256extend downwardly through the beams, as illustrated in FIG. 13H, toisolate the silicon of the beams 6276 and 6278 from the silicon of thesubstrate 6202. The metal pads 6230 and 6232 are connected to the beamsat selected via locations by means of interconnects 6244 and 6246,resulting in multiple conduction paths or multiple connections to themicrostructure. For one embodiment the beams 6276 and 6278 are a part ofa larger micromechanical structure with an array of similar beams andinterconnects, and are intended only to represent the isolation process.The sidewall films 6274 can remain on the microstructures or be removedby an isotropic dielectric etch. In general, the sidewall passivationfilm can be removed if its presence affects the behavior of themiocromechanical structure.

There are numerous alternative variations that could be used for theoperations used to fabricate the blade actuator and the associatedframes and stages. For example, the aluminum metallization typicallyused for routing voltages to various blades could be made from othermetals, such as copper, tungsten, or titanium. The isolation joints usedto electrically isolate regions of the frames and stages are typicallymade from silicon dioxide, but could be made from silicon nitride,borophosphosilicate glass (“BPSG”), or combinations or silicon nitrideand polysilicon. The isolation joints need to electrically isolateregions of the frames and stages for one to be able to apply theappropriate voltages to the blade actuators. Various materials could beused to achieve that result. With respect to the silicon used to formthe blade actuator, the underlying requirement is a conductive materialthat holds the same shape. Currently silicon is a convenient materialgiven the existing fabrication tools that are common to thesemiconductor industry. Nevertheless, other materials meeting theunderlying requirement can be used for alternative embodiments.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

What is claimed is:
 1. An apparatus, comprising: a stage having anundersurface; a first blade coupled to the stage, the first bladeextending perpendicularly from the undersurface of the stage; a framehaving an undersurface, the stage pivotally coupled to the frame; and asecond blade coupled to the frame, the second blade extendingperpendicularly from the undersurface of the frame, the second bladebeing in proximity with the first blade.
 2. The apparatus of claim 1,wherein the first blade is configured to move relative to the secondblade along a rotational range of motion.
 3. The apparatus of claim 2,wherein a gap between the first blade and the second blade is maintainedsubstantially constant throughout the rotational range of motion.
 4. Theapparatus of claim 3, wherein each of the first and second blades has alength and a height, and wherein the range of motion is determined bythe length and the height of the first and second blades.
 5. Theapparatus of claim 3, wherein the first blade is configured to moverelative to the second blade in the presence of a potential between thefirst and second blades.
 6. The apparatus of claim 3, wherein the firstblade has a length and wherein the range of motion of the first blade isdetermined by the length.
 7. The apparatus of claim 1, wherein the firstblade has a first length, the second blade has a second length, andwherein the first blade is tapered.
 8. The apparatus of claim 1, whereinthe first blade has a height extending from the undersurface of theframe, a length extending from a first side to a second side, andwherein the first side has a first width and the second side has asecond width different from the first width.
 9. The apparatus of claim1, wherein the stage is pivotally coupled to the frame with a flexure.10. The apparatus of claim 9, wherein the flexure is configured torotate about a single axis and substantially restrict rotation aboutother axes, the single axis residing along a length of the flexure. 11.The apparatus of claim 10, further comprising a third blade coupled tothe frame, the third blade extending perpendicularly from theundersurface of the frame, the third blade being in proximity with thefirst blade.
 12. The apparatus of claim 11, wherein the second blade isconfigured to rotate the stage in one direction and the third blade isconfigured to rotate the stage in an opposite direction.
 13. Theapparatus of claim 9, wherein the flexure comprises a plurality oftorsion beams.
 14. The apparatus of claim 13, wherein the plurality oftorsion beams are substantially parallel to one another.
 15. Theapparatus claim 13, wherein each of the plurality of torsion beams has alength and wherein the plurality of torsion beams are non-parallel alongportion of the lengths.
 16. The apparatus of claim 1, wherein the firstblade is electrically isolated from the second blade.
 17. The apparatusof claim 1, further comprising: a first set of one or more additionalblades coupled to the stage, the first set of one or more additionalblades electrically connected to each other and to the first blade; anda second set of one or more additional blades coupled to the stage, thesecond set of one or more additional blades electrically connected toeach other and to the second blade.
 18. The apparatus of claim 1,wherein the first blade extends, at rest, substantially perpendicular toan initial lateral direction of motion of the first blade.
 19. Theapparatus of claim 1, wherein the first blade has a longest dimensionextending perpendicularly from the undersurface of the stage.
 20. Anapparatus, comprising: a frame having an undersurface; a stage pivotallycoupled to the frame, wherein the stage has an undersurface; a firstblade coupled to the stage, wherein the first blade extendsperpendicularly from the undersurface of the stage; and a second bladecoupled to the frame, wherein the second blade extends perpendicularlyfrom the undersurface of the frame, the second blade configured to moverelative to the first blade through a rotational range of motion, thesecond blade remaining spaced apart from but in proximity with the firstblade throughout the rotational range of motion.
 21. The apparatus ofclaim 20, wherein the first blade has a tapered length.
 22. Theapparatus of claim 20, wherein the stage is pivotally coupled to theframe with a flexure.
 23. The apparatus of claim 22, wherein the flexurecomprises a plurality of torsion beams.
 24. The apparatus of claim 20,further comprising a third blade coupled to the frame, the third bladeextending perpendicularly from the undersurface of the frame, the thirdblade being in proximity with the first blade.
 25. The apparatus ofclaim 22, further comprising isolation segments coupled to electricallyseparate the first, second and third blades.
 26. The apparatus of claim22, further comprising a third blade coupled to the frame, the thirdblade extending perpendicularly from the undersurface of the frame, thethird blade being in proximity with the first blade.
 27. The apparatusof claim 25, wherein the second blade is configured to rotate the stagein one direction and the third blade is configured to rotate the stagein an opposite direction.
 28. The apparatus of claim 25, wherein thefirst blade is configured to pass by the second blade when the flexureis rotated in the one direction and wherein the first blade isconfigured to pass by the third blade when the flexure is rotated in theopposite direction.
 29. An apparatus, comprising: a central stage; amovable frame disposed around the central stage; a fixed frame disposedaround the movable frame; a first blade coupled to the central stage,the first blade extending perpendicularly from an undersurface of thecentral stage; and a second blade coupled to the movable frame, thesecond blade extending perpendicularly from an undersurface of themovable frame, the second blade being in proximity with the first blade.30. The apparatus of claim 29, wherein the first blade is configured tomove relative to the second blade along a rotational range of motion.31. The apparatus of claim 29, wherein a gap between the first blade andthe second blade is maintained substantially constant throughout a rangeof motion.
 32. The apparatus of claim 30, further comprising a mirrorcoupled to the central stage.
 33. The apparatus of claim 29, wherein thecentral stage is coupled to the movable frame with a first flexure andthe movable frame is coupled to the fixed frame with a second flexure,the second flexure being orthogonal to the first flexure.
 34. Theapparatus of claim 33, wherein the first flexure comprises a pair ofparallel torsion beams.
 35. The apparatus of claim 33, wherein thesecond flexure comprises a pair of non-parallel torsion beams.
 36. Theapparatus of claim 33, wherein the movable frame comprises: a main bodycoupled to the second flexure; an end bar coupled to the first flexure;and a support member coupled between the main body and the end bar. 37.The apparatus of claim 36, wherein the support member is coupled to themain body at a non-perpendicular angle.
 38. The apparatus of claim 29,further comprising: a first flexure coupling the control stage to themovable frame; a second flexure coupling the fixed frame to the movableframe; a third blade coupled to movable frame, the third blade extendingperpendicularly from the undersurface of the movable frame; a fourthblade coupled to the fixed frame, the fourth blade extendingperpendicularly from an undersurface of the fixed frame, wherein thefourth blade is in proximity with the third blade.
 39. The apparatus ofclaim 38, wherein the movable frame has a top surface and wherein theapparatus further comprises an electrical trace coupled to the thirdblade, wherein the electrical trace resides on the top surface of themovable frame and on the second flexure.
 40. The apparatus of claim 38,wherein an electrical potential between the first and second bladesresults in motion between the stage and the movable frame, whichcomprises a first degree of freedom, wherein an electrical potentialbetween the third and fourth blades results in motion between themovable frame and the fixed frame, which comprises a second degree offreedom.
 41. An apparatus, comprising: a stage having an undersurface; afirst blade coupled to the stage, the first blade extendingperpendicularly from the undersurface of the stage; a frame having anundersurface, the stage pivotally coupled to the frame with a flexurehaving a plurality of torsion beams; and a second blade coupled to theframe, the second blade extending perpendicularly from the undersurfaceof the frame, the second blade being in proximity with the first blade,wherein a gap is maintained between the first and second bladessubstantially uniform when an electrostatic potential is applied betweenthe first and second blades.
 42. An apparatus, comprising: a pluralityof actuators, each of the plurality of actuators comprising: a centralstage; a movable frame disposed around the central stage; a first bladecoupled to the central stage, the first blade extending perpendicularlyfrom an undersurface of the central stage; and a second blade coupled tothe movable frame, the second blade extending perpendicularly from anundersurface of the movable frame, the second blade being in proximitywith the first blade; and a fixed frame disposed around each movableframe of the plurality of actuators.
 43. The apparatus of claim 42,wherein the fixed frame comprises a plurality of isolation segments.